1. Field of the Invention
The invention concerns electrically programmable memories (EPROMs). It may also concern electrically erasable programmable memories.
2. Description of the Prior Art
Memory cells commonly used today consist of a floating gate transistor with its gate connected to a word line, its drain to a bit line and its source to a reference potential which is generally the electrical ground of the circuit.
A word line is connected to the gates of all the transistors of one and the same line. A bit line is connected to drains of all the transistors of one and the same column. A cell is addressed by the selection of a bit line and a word line.
The cell is programmed by the injection of electrical charges into the floating gate, by applying a relatively high potential to the drain and to the gate, the source being grounded.
The state of the cell (i.e. whether it is programmed or not) is read by pre-charging the drain, hence the bit line, at a low voltage and then applying a potential to the gate and detecting the discharge current of the bit line. If this current is high, it means that the cell can easily be made conductive by the read potential. This means that it is not programmed. If the current is very low, it means that the cell has not been made conductive by the read potential applied to the gate. This means that it has been programmed. A threshold comparator detects the current and gives a binary output signal indicating the state of the cell.
A major problem encountered in the manufacture and programming of memories of this type relates to the testing of the memory after programming. It has to be ascertained that the state of the cells is really that which is desired, i.e. that all the cells which have to be programmed, have been programmed, and that those that have to stay blank (i.e. non-programmed) have remained blank.
The simplest test consists in performing a systematic operation to read the state of the cells one after the other, under the normal conditions in which the memory is read, i.e. in performing the test while applying the supply potentials specified for the memory in read mode. By this test, a binary piece of information is collected on the state of each cell.
However, it has been realized that the programming state of a cell is not purely a binary piece of information. The cell may be programmed to a "stronger" or "weaker" extent i.e. the quantity of charges trapped in the floating gate may be greater or smaller.
The present trend is to program the cells very "strongly" for several reasons: a first reason is the fact that the quantity of charges trapped does not remain constant forever. There is a certain degree of loss of charges in time. The information is therefore retained for a limited lifetime which it is desired to increase as far as possible. Hence, the quantity of trapped charges is increased at the outset. A second reason is the fact that there is no precise knowledge of the quantity of charges trapped during a programming operation. The programming voltage and duration of the programming pulse are known, but these two parameters must be overestimated on an a priori basis in order to be certain that, even in the most unfavorable cases, the quantity of charges trapped will be sufficient. A third reason is the fact that the user of the memory must be allowed a certain range of variation in the read voltage applied to the word lines. The nominal voltage is, for example, five volts but the reading should be accurate if, for one reason or another, the user applies a slightly greater voltage (for example, six volts). However, the greater the voltage, the greater will be the extent to which the cell will let through current, especially if it is weakly programmed. There is thus a risk that a programmed cell will be considered by the read comparator to be a non-programmed cell, and this is unacceptable.
For all these reasons, the value of the programming voltage and the period for which this voltage is applied during the programming stage are increased. However, increasing the voltage implies oversizing all the circuits which will have to bear this voltage. The result of this is that additional space is taken up on the surface of the integrated circuit. Besides, increasing the duration of the programming stage is a very cumbersome task for memories having a large number of cells, since the programming is done cell by cell (or word by word for memories organized in words).
Since the current flowing through the cell is related to the "stronger" or "weaker" degree to which this cell has been programmed, it may be sought to measure this current, precisely in order to obtain a piece of information on the quantity of trapped charges. This will provide greater efficiency in the subsequent adjustment of the programming conditions or, again, it will enable the reprogramming of cells seen to be insufficiently programmed.
This current can be measured by a test with probe tips during the manufacture of the memory while this memory is still in the state of a semiconducting slice and has not yet been cut out into individual, integrated circuit chips nor encapsulated in a package. This test uses specific circuits and specific testing pads in the integrated circuit. Of course, this test cannot be performed with probe tips, if the memory is programmed only after it has been encapsulated in a enclosed package.
When the memory is encapsulated, the specific testing pads are no longer accessible. The only accessible elements are the connection terminals needed by the user of the memory (supply terminals, addressing terminals and data output terminals) for it is sought to minimize the number of connection terminals for reasons of space and cost.
This is why, until now, the task has been limited to a binary test of the state of the cells of an encapsulated memory to check whether they are programmed or not, without its being really possible to achieve a measurement of programming quality for each of the cells.
An object of the invention is to propose an improved test procedure which resolves these problems and which makes it possible not only to ascertain that the cells are programmed or not programmed but also, in particular:
to give a better definition of the ranges of read voltage values for which the read comparator will give a precise indication on the state of the cells;
to give a better definition of the voltages and programming times sufficient to program the cells with a desired retention period.